BSN: Remove db lock and add missing contexts

Adds context tracking decorators that were missing
from router interface methods. Without them, new
sessions were being created instead of using the
existing context which was causing transaction
issues.

Modifies the servermanager to store context references
as weakrefs so if multiple functions are called before
the rest functions are called, the first one doesn't steal
the only context reference with a pop() call.

Removes a DB lock for update in the server manager that occured
during rest calls that was triggering deadlocks due to the
file lock synchronization for the rest calls.

Closes-Bug: #1336251
Change-Id: Iad3d61e2c23832b3ad760a999fbab7feaa13f805

neutron/plugins/bigswitch/plugin.py

@@ -968,6 +968,7 @@ class NeutronRestProxyV2(NeutronRestProxyV2Base,
             self.servers.rest_delete_router(tenant_id, router_id)
             return ret_val
 
+    @put_context_in_serverpool
     def add_router_interface(self, context, router_id, interface_info):
 
         LOG.debug(_("NeutronRestProxyV2: add_router_interface() called"))
@@ -996,6 +997,7 @@ class NeutronRestProxyV2(NeutronRestProxyV2Base,
                                                    intf_details)
             return new_intf_info
 
+    @put_context_in_serverpool
     def remove_router_interface(self, context, router_id, interface_info):
 
         LOG.debug(_("NeutronRestProxyV2: remove_router_interface() called"))
@@ -1087,6 +1089,7 @@ class NeutronRestProxyV2(NeutronRestProxyV2Base,
             else:
                 self._send_floatingip_update(context)
 
+    @put_context_in_serverpool
     def disassociate_floatingips(self, context, port_id):
         LOG.debug(_("NeutronRestProxyV2: diassociate_floatingips() called"))
         super(NeutronRestProxyV2, self).disassociate_floatingips(context,